The present invention relates to the synthesis of hydrogen peroxide, and more particularly, to the synthesis of hydrogen peroxide in which the use of organic solvents is reduced or eliminated.
Hydrogen peroxide (H2O2) is often considered to be a xe2x80x9cgreenxe2x80x9d material, in that it is increasingly used to replace chlorine-containing reagents in paper bleaching and in water purification. For this reason, as well as others, hydrogen peroxide production is estimated to increase steadily through the beginning of the next century.
The production of hydrogen peroxide is a mature process in that the general procedure has not changed appreciably in twenty years. Indeed, recent research publications in the area of hydrogen peroxide synthesis are somewhat scarce. Typically, hydrogen peroxide is generated in a two-step process, wherein hydrogen is first reacted with a 2-alkyl anthraquinone (usually 2-ethyl or 2-amyl anthraquinone) in an organic solvent to produce the corresponding tetrahydroquinone (2-alkyl tetrahydroquinone) The reaction is catalyzed by a simple palladium-on-alumina catalyst. Conditions for this reaction are typically 30 to 70xc2x0 C. with hydrogen pressures up to 300 psi. Given the nature of the reactants, the reactor contains three phases (gas, liquid, and solid catalyst) and previous work has shown that the reaction is completely mass transfer limited, such that the rate of the reaction is essentially the rate at which hydrogen diffuses into the liquid phase. Partly as a result of this inefficiency of hydrogen use, side reactions (hydrogenation of one or both of the aromatic rings) also occur, and byproducts build up during repeated cycling of the anthraquinone. These byproducts must periodically be removed and treated. The organic solvent employed is typically a mixture of an aromatic (a good solvent for the anthraquinone) and a long-chain alcohol (a good solvent for the hydroquinone).
The second step of the process involves oxidation of the hydroquinone, regenerating the anthraquinone and producing hydrogen peroxide. Here the catalyst is retained in the first reactor, and the solution of alkyl anthraquinone, alkyl tetrahydroquinone and organic solvent (the working solution) is transferred to the second reactor, where the hydroquinone is reacted with oxygen (as air or oxygen). This reaction is uncatalyzed. Similar to the first reaction, the second reaction is mass transfer limited by the rate at which oxygen can diffuse from the gas to liquid phases. Finally, the hydrogen peroxide is stripped from the organic solvent via liquid-liquid extraction with water and sold as an aqueous mixture (usually 30 to 50%).
Because the final step in the production of hydrogen peroxide involves a liquid-liquid extraction between aqueous and organic phases, the final product is contaminated to some extent by the organic phase. Given that H2O2 is promoted as a green reagent for paper production, and is also used in water purification, the organics in the final product must be minimized. Significant effort is thus made to strip the organic contaminants from the product.
Although approximately 95% of the world""s hydrogen peroxide is produced via the anthraquinone process described above, a number of other synthetic routes exist. For example, from the 1960""s to the 1980""s, Shell maintained several hydrogen peroxide production plants that employed a free-radical initiated oxidation of a secondary alcohol (isopropanol) for the generation of hydrogen peroxide. These plants were closed, however, in the early 1980""s because they could not compete economically with the well-known anthraquinone route to hydrogen peroxide production. The primary disadvantages to the use of secondary alcohol oxidation are that (a) one has to distill a complex mixture of hydrogen peroxide, water, residual alcohol, and the ketone byproduct of the reaction to purify the hydrogen peroxide product, and hot hydrogen peroxide is a safety hazard; and (b) the required reaction temperature for this process is rather high, 100 to 150xc2x0 C., also a safety hazard. During the 1980""s, Arco Chemical explored the use of another secondary alcohol, phenethyl alcohol, for use in the production of hydrogen peroxide. This secondary alcohol exhibited better reactivity than isopropanol, but that process suffers from similar disadvantages to the isopropanol process described above.
Arguably, the ideal synthetic route for producing hydrogen peroxide would be one that employs the simple reaction of hydrogen plus oxygen, yet which could also run safely. Clearly, a mixture of hydrogen and oxygen can pose a serious safety hazard, one fact that has prevented such a technology from being scaled up and commercialized to date. On the other hand, production of hydrogen peroxide from only oxygen and hydrogen would represent the most efficient (and thus the most inexpensive) and cleanest method by which to generate the product.
Indeed, a number of research groups throughout the world have been investigating a more direct route to the production of hydrogen peroxide, that is, via the direct reaction of hydrogen and oxidation. The keys to a successful process include (a) maintaining safe operating conditions, (that is, preventing explosion), (b) generating hydrogen peroxide continuously and at high rates (to satisfy economic constraints), and (c) preventing decomposition of the hydrogen peroxide product once it is formed. To date, attempts to develop a commercially viable synthetic route to hydrogen peroxide via the direct route of hydrogen and oxidation have met with very limited success.
It remains, therefore, very desirable to develop reactants and processes for the synthesis of hydrogen peroxide.
A method for synthesizing hydrogen peroxide using a CO2-philic anthraquinone is described in U.S. patent application Ser. No. 09/106,480, filed Jun. 29, 1998, U.S. Pat. No. 6,342,196, and entitled SYNTHESIS OF HYDROGEN PEROXIDE, assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference. That method comprises generally the steps of:
synthesizing an analog of anthraquinone that is miscible with (in the case of a liquid analog) or soluble in (in the case of a solid analog) carbon dioxide;
reacting the analog of anthraquinone with hydrogen in carbon dioxide to produce a corresponding analog of tetrahydroquinone; and
reacting the analog of tetrahydroquinone with oxygen to produce the hydrogen peroxide and regenerate the analog of anthraquinone.
Preferably, the regenerated analog of anthraquinone is recycled for future use.
The step of synthesizing an analog of anthraquinone that is miscible in carbon dioxide preferably comprises the step of attaching to anthraquinone at least one modifying or functional group that is relatively highly soluble in CO2 (xe2x80x9cCO2-philicxe2x80x9d). The miscibility/solubility of the resulting analogs of anthraquinone are several orders of magnitude greater at the operating pressures of the present invention than the solubility of 2-alkyl anthraquinone in carbon dioxide at pressures equal to or below 5000 psi. Alkyl-anthraquinones used in the commercial synthesis of hydrogen peroxide do not exhibit appreciable solubility in carbon dioxide at pressures below 5000 psi. In that regard, a number of studies have explored the solubility of alkyl-functional anthraquinones in carbon dioxide and found generally that the system exhibits solid-fluid phase behavior with maximum solubilities of approximately 10xe2x88x922 mM. See, for example, Joung, S. N., Yoo, K. P., J. Chem. Eng. Data, 43, 9 (1998). Coutsikos, P., Magoulos, K., Tassios, D., J. Chem. Eng. Data, 42, 463 (1997). Swidersky, P., Tuma, D., Schneider, G. M., J., Supercrit. Fl., 9, 12 (1996). ibid, 8, 100 (1995).
Preferably, the CO2-philic functionalized anthraquinones and the corresponding hydroquinones exhibit reactivity similar to the 2-alkyl anthraquinone and hydroquinones used in the current commercial synthesis of hydrogen peroxide. Indeed, the kinetic rate constants calculated for the oxygenation of the functionalized anthraquinones were found to be approximately ten time greater than anthraquinone. The use of CO2-philic groups to increase the solubility of a molecule in carbon dioxide is also discussed in U.S. Pat. No. 5,641,887, the disclosure of which is incorporated herein by reference.
In general, the analog of anthraquinone preferably has the formula: 
At least one of R1, R2, R3, R4, R5, R6, R7, and R8 (corresponding to the 1, 2, 3, 4, 5, 6, 7, and 8 carbons on the anthraquinone ring structure) is a modifying group or functional group that is miscible/soluble in carbon dioxide. Attachment of one or more such CO2-philic groups to anthraquinone and other compounds results in an analog of anthraquinone and such other compounds that is miscible/soluble in carbon dioxide. In that regard, R1, R2, R3, R4, R5, R6, R7, and R8 are preferably, independently, the same or different, H, RC or RSRC, wherein RS is a connector or a spacer group and RC is a fluoroalkyl (fluorinated alkyl) group, a fluoroether (fluorinated ether) group, a silicone group, an alkylene oxide group, a phosphazene group or a fluorinated acrylate group. At least one of R1, R2, R3, R4, R5, R6, R7, and R8 is not H. Preferably, RC is a fluoroalkyl group, a fluoroether group or an alkylene oxide group. More preferably, RC is a fluoroether group or an alkylene oxide group.
The spacer group, RS, when present, can simply be a connective group used to attach a CO2-philic group to anthraquinone or can additionally act to space the CO2-philic group away from the anthraquinone. The spacer group is preferably a group which provides a simple synthetic route to achieve the desired analog of anthraquinone without substantially adversely affecting the miscibility of the analog of anthraquinone in carbon dioxide or the reactivity of the analog of anthraquinone and the corresponding hydroquinone in the synthesis of hydrogen peroxide. For example, the spacer group can be an alkylene group, an amino group, an amido group, an ester group or an alkyl ester group. As used herein in connection with RS, the term xe2x80x9calkylene groupxe2x80x9d refers to a linear or branched alkylene group. A linear alkylene group, for example, has the formula xe2x80x94(CH2)nxe2x80x94. As used herein in connection with RS, the term xe2x80x9camino groupxe2x80x9d refers to a secondary amino group having the formula xe2x80x94NHxe2x80x94 or a tertiary amino group having the formula xe2x80x94NR11Hxe2x80x94, wherein R11 can generally be any substituent that doesn""t interfere with the reactivity of the desired analog. For example, R11 can be an alkyl group. As used herein in connection with RS, the term xe2x80x9camido groupxe2x80x9d refers to secondary amido having the formula xe2x80x94NHCOxe2x80x94, or a tertiary amido group having the formula xe2x80x94NR11COxe2x80x94 wherein R11 is as defined above. As used herein in connection with RS, the term xe2x80x9cester groupxe2x80x9d refers to a group having the formula xe2x80x94OCOxe2x80x94. As used herein in connection with RS, the term xe2x80x9calkyl ester groupxe2x80x9d refers to a group having the formula xe2x80x94R12OCOxe2x80x94, wherein R12 is an alkyl group. The spacer group itself need not be CO2-philic. If it is desired to use the spacer group to space the CO2-philic group away from the anthraquinone ring structure, an alkylene group is preferably used, either alone or in combination with another connective group.
The total molecular weight of the CO2-philic groups RC attached to the analog of anthraquinone is preferably between approximately 200 and approximately 7500 to make the analog of anthraquinone miscible/soluble in carbon dioxide. One or more CO2-philic groups can be attached to the anthraquinone ring structure. For example, each of R2, R3, R6, and R7, can comprise a perfluoroalkyl group having a molecular weight of 50. More preferably, the total molecular weight of the CO2-philic groups is between approximately 500 and approximately 5000. Most preferably, the total molecular weight of the CO2-philic groups is between approximately 500 and approximately 1500.
The fluoroalkyl groups of the present invention are preferably linear perfluoroalkyl groups comprising the formula/repeat group:
xe2x80x94(CF2)gxe2x80x94.
wherein g is an integer.
The fluoroether groups of the present invention are preferably perfluorinated and comprise the formula/repeat group: 
wherein each of x, y and z is an integer greater than or equal to 0 and at least one of x, y and z is not equal to 0.
The silicone groups of the present invention preferably comprise the formula/repeat group(s): 
wherein R9 and R10 are chosen to not substantially affect the CO2-philic nature of the silicone group or the reactivity of the functionalized analogs of anthraquinone. R9 and R10 may, for example, be, independently, the same or different, H, an alkyl group, an aryl group, an alkenyl group, or an alkoxyl group. In the above formula, b is an integer. Preferably, R9 and/or R10 is a fluoroalkyl group.
The alkylene oxide groups of the present invention preferably comprise the formula/repeat group: 
wherein d is an integer and e is an integer.
The fluorinated acrylate groups of the present invention preferably comprise the formula/repeat group: 
wherein g and j are integers.
The phosphazine groups of the present invention preferably comprise the formula/repeat group: 
wherein m is an integer and R9 and R10 are as defined above.
The oxidation of the hydroquinone preferably takes place in carbon dioxide at substantially the same pressure as the hydrogenation reaction. The hydrogen peroxide product is preferably recovered via a liquid-liquid extraction between the carbon dioxide phase and an aqueous phase. The liquid-liquid extraction is preferably conducted without significantly reducing the operating pressure. Likewise, the carbon dioxide is preferably recycled to the extractor without a significant drop in pressure. Such a process for separation/recovery of hydrogen peroxide product avoids the high costs associated with recompression, while taking full advantage of carbon dioxide""s green properties in running a contamination-free liquid-liquid extraction between a carbon dioxide phase and an aqueous phase.
Moreover, using carbon dioxide as the solvent for the process allows one to generate a single phase system of hydrogen plus anthraquinone (for the first reaction of the synthesis), or oxygen plus tetrahydroanthraquinone or tetrahydroquinone (for the second reaction of the synthesis). It is known that hydrogen is completely miscible with carbon dioxide above a temperature of approximately 31xc2x0 C. Hydrogen and carbon dioxide have been found to not form separate phases under the operating conditions of the present invention. The reactions can thus be carried out without the mass transfer limitation of the current commercial process for the synthesis of hydrogen peroxide, suggesting that one could operate more efficiently, using less hydrogen and/or at lower temperatures, while producing fewer byproducts.
Furthermore, the operating pH for the stripping operation to recover the hydrogen peroxide from the organic phase into the aqueous stream in the current commercial process for the synthesis of hydrogen peroxide is preferably approximately 3.0 to partition the hydrogen peroxide into the aqueous phase. Because the carbon dioxide dissolves in water to form carbonic acid, the pH of the water in the presence of high pressure carbon dioxide is approximately 3.0, assisting in partitioning the hydrogen peroxide into the aqueous phase.
The present inventors have discovered that synthetic routes to the production of hydrogen peroxide other than via CO2-philic anthraquinone can be modified to take place in a carbon dioxide phase. For example, the present invention provides a method for synthesizing hydrogen peroxide, comprising generally the step of mixing an analog of a secondary alcohol that is miscible with or soluble in carbon dioxide with a free radical initiator and oxygen in carbon dioxide (preferably liquid or supercritical carbon dioxide) to generate hydrogen peroxide. The free radical initiator may, for example be a peroxide. Preferably, the free radical initiator is hydrogen peroxide. The free radical initiator is preferably present in an amount less than approximately 1 wt % of the analog of the secondary alcohol. The reaction preferably takes place in a pressure range of approximately 900 psi to approximately 2500 psi. The reaction also preferably takes place in a temperature range of approximately 20xc2x0 C. to approximately 100xc2x0 C.
The analog of a secondary alcohol may, for example, have the formula: 
wherein R13, R14, R15, R16, and R17 are independently, the same or different, H, RC or RSRC, wherein RS is a spacer group and RC is a CO2-philic group, and wherein at least one of R13, R14, R15, R16, and R17 is not H, and R18 is an alkyl group (for example, a methyl group). Two or more CO2-philic groups can be present. The CO2-philic groups and spacer groups are generally as described above. The CO2-philic groups preferably comprise a fluoroalkyl group, a fluoroether group, a silicone group, an alkylene oxide group, a fluorinated acrylate group, or a phosphazine group. As also described above, the spacer group may be an alkylene group, an amino group, an amido group, an alkyl ester group or an ester group. For example, the spacer group may be xe2x80x94NHCOxe2x80x94, xe2x80x94NCH2COxe2x80x94 or xe2x80x94CH2OCOxe2x80x94.
Preferably R13 and R15 are H in the above compound. Limiting the CO2-philic substituents to the 3 and 5 position on the aromatic ring (that is, R14 and R16) is preferred for minimization of byproduct formation during hydrogenation.
The secondary alcohol may also have the formula R11CH(OH)R18 wherein R11 is a CO2-philic group and R18 is an alkyl group (for example a methyl group). Aromatic secondary alcohols typically exhibit greater reactivity, however.
The method preferably also comprises the step of regenerating the analog of the secondary alcohol by hydrogenating the corresponding CO2-philic ketone produced in the reaction. For example, the CO2-philic ketone may be cycled to a hydrogenation reactor where the secondary alcohol is regenerated.
The reaction of the analog of the secondary alcohol with the free radical initiator and oxygen may take place in the presence of a catalyst. A catalyst is not necessary, however. Preferably, a catalyst, when present, is miscible in or soluble in carbon dioxide. The catalyst may, for example, have the formula: 
wherein R13, R14, R15 and R16, are independently, the same or different, H, RC or RSRC, wherein RS is a spacer group and RC is a CO2-philic group as described above. Although not necessary, the catalyst is preferably CO2-philic. Therefore, at least one of R13, R14, R15 and R16 is preferably not H. Two or more CO2-philic groups may be present. In the case that the catalyst is not CO2-philic, the catalyst will be heterogeneous. Use of a heterogeneous catalyst may facilitate keeping the catalyst in the oxidation reactor.
The present invention also provides a chemical compound having the formula: 
wherein R13, R14, R15, R16, R17 and R18 are as described above.
The present invention also provides a compound having the formula: 
wherein R13, R14, R15 and R16 are as described above.
Still further, the present invention provides a compound having the formula R13CH(OH)R18 wherein R13 and R18 are as described above.
The present invention also provides a method for synthesizing hydrogen peroxide, comprising the steps of:
mixing hydrogen, oxygen and a CO2-philic catalyst in carbon dioxide (preferably liquid or supercritical carbon dioxide), the CO2-philic catalyst being soluble or miscible in carbon dioxide and being suitable to catalyze the reaction of hydrogen and oxygen (in the carbon dioxide phase) to produce hydrogen peroxide; and
reacting hydrogen and oxygen to produce hydrogen peroxide. The method preferably further comprises the step of extracting the hydrogen peroxide product into an aqueous phase.
The method may also comprise the step of creating a second phase, which is an aqueous phase, in contact with the carbon dioxide phase to create a biphasic system. In this embodiment, the hydrogen peroxide product preferentially partitions into the aqueous phase.
The reaction preferably takes place in a pressure range of approximately 900 psi to approximately 2500 psi. The reaction also preferably takes place in a temperature range of approximately 20xc2x0 C. to approximately 100xc2x0 C.
The CO2-philic catalyst may have the formulas M(L)rXt, wherein M is a group 8, 9 or 10 metal, L is a CO2-philic ligand, X is a halogen, r is an integer between 1 and 3 and t is an integer between 1 and 2. Preferably, M is Pd. L may, for example, be P(RCxe2x80x94C6H4)3 or P(RCR19)3, wherein R19 is an alkyl group and wherein RC is a CO2-philic group as described above. RC may, for example, be 1H, 1H, 2H, 2H-perfluorooctyl(xe2x80x94(CH2)2(CF2)6F). Preferably, X is Cl.
As discussed above, the CO2-philic analog compounds of the present invention are typically several orders of magnitude more soluble in or miscible with carbon dioxide than the corresponding underivitized compounds, while retaining their reactivity.